Hidden Pairs
Short answer: a hidden pair is two shapes whose only home in a unit is the same two cells — so those two cells must hold that pair, and every other candidate in them can be erased. The pair is "hidden" because it sits camouflaged among extra notes until you go looking for it.
What a hidden pair looks like
Don't ask what a cell can hold — ask where a shape can go. Run through the shapes still missing from a unit and tally the cells each one can still occupy. When two different shapes both point to the very same two cells, those cells belong to that pair, whatever else is pencilled inside them.
Unlike a naked pair, the two cells here are not down to two candidates on their own — they carry more. The move is to see the pair hiding among that clutter and clear everything else out of the two cells.
How to find a hidden pair
- Pick a unit and list what is missing. Choose a row, column or box and note which shapes it still needs.
- Map each shape to its possible cells. For each missing shape, mark which empty cells in the unit could still take it.
- Find two shapes sharing two cells. Look for two shapes whose possible cells are the same two — and only those two. That is the hidden pair.
- Strip the other candidates. Those two cells must hold the pair, so erase every other candidate from them. Now they behave like a naked pair for the rest of the unit.
A hidden pair is often the same position a naked pair would reach — you just get there by tracking shapes instead of cells. Extend the idea to three shapes in three cells and you have a hidden triple.
Hidden pair vs naked pair
Two views of one pattern. A naked pair is obvious — two cells showing only the pair. A hidden pair is disguised — the same two shapes are confined to two cells, but those cells wear extra candidates you have to clear away first. Track cells for naked, track shapes for hidden — exactly like hidden and naked singles.
Frequently asked questions
What is a hidden pair in sudoku?
A hidden pair is two shapes that can legally go in only the same two cells of a row, column or box. Those two cells must hold that pair, so any other candidates in them can be eliminated — even though the cells looked busy before.
Why is it called "hidden"?
Because the pair is buried among other candidates. Unlike a naked pair, the two cells are not already down to just those two shapes — you have to notice that the pair has nowhere else to go before you can clear the clutter.
What does finding a hidden pair actually do?
It removes candidates, not a placement. After you strip the extra notes from the two cells, the pair frequently turns into a naked single or opens one nearby, which is what you place next.
Are hidden pairs harder than naked pairs?
A little, because they are camouflaged. Naked pairs jump out of the pencil marks; hidden pairs need you to track where each missing shape can still go. The underlying logic is identical.